DE 197 57 504 B4 describes an adjustable camshaft for the valve gear of an internal combustion engine, with an outer shaft and an inner shaft which extends through the outer shaft. Cams are arranged on the outer shaft rotationally fixedly therewith, and further cams are connected rotationally fixedly to the inner shaft. Oil is conducted into the gap between the inner shaft and the outer shaft via a central bore in the inner shaft, and the oil passes into the gap between the inner shaft and the outer shaft via radially running openings from the central bore in the inner shaft.
DE 10 2005 014 680 A1 shows a further adjustable camshaft for the valve gear of an internal combustion engine, with an outer shaft on which at least one first cam is arranged and connected rotationally fixedly thereto, and with an inner shaft which extends through the outer shaft and to which at least one second cam is connected rotationally fixedly. In order to deliver oil for lubrication into the gap between the inner shaft and the outer shaft, a supply channel is shown in the outer shaft for example, located in a portion of the outer shaft into which the inner shaft does not extend. Thus oil can reach into the gap between the inner shaft and the outer shaft through a filter, shown as an example, wherein for example an oil injection nozzle may be used as an oil supply device.
By the introduction of oil into the gap between the inner shaft and the outer shaft, reliable lubrication can be ensured between the inner shaft and the outer shaft, but the bearing arrangement of the second cam connected to the inner shaft on the outside of the outer shaft may not be supplied with sufficient lubricating oil. This inadequate oiling state of the rotatable cam arranged on the outer shaft can result in increased wear, in particular below the cam crown, which can lead to premature failure of the camshaft. This can be remedied by hardening the surface of the outer shaft in order to prevent the abrasion of the surface, wherein the hardening of the surface is however associated with great technical complexity and entails further costs.